This invention relates to high-strength low-alloy (HSLA) steels, and in particular, to a method of making an HSLA hot rolled steel having a unique composition of alloying elements and high yield strength.
High-strength low-alloy steels are a group of steels intended for general structural or miscellaneous applications and have specified minimum yield points above 40,000 pounds per square inch (40 ksi). These steels typically contain small amounts of alloying elements to achieve their strength in hot-rolled or other normalized conditions. HSLA steels are available as sheet, strip, plates, bars and shapes. These steels are generally sold as proprietary grades. Advantageous characteristics of all-purpose HSLA steel include high strength, good formability, good weldability, and good toughness. In general, HSLA steel products are stronger and tougher than products made from structural carbon steel. HSLA steels also offer a high fatigue resistance to repeated loading, high abrasion resistance, and superior resistance to atmospheric corrosion.
Typical application areas for HSLA steels include mobile crane supports, earth moving equipment, truck rails, automobile parts, railroad freight cars and welded beams. HSLA steels can generally be used advantageously in any structural application in which their greater strength can be utilized either to decrease the weight or increase the durability of the structure.
A number of different compositions of HSLA steels containing various alloying elements have been developed which offer combinations of other properties and characteristics in addition to increased strength. Regardless of the composition of alloying elements used, the strength of an HSLA steel is primarily determined by its microstructures. HSLA steels conventionally have a ferrite-pearlite microstructure. In addition, some HSLA steels have been developed with a ferrite-bainite microstructure.
In an HSLA steel with a ferrite-bainite microstructure, a number of strengthening mechanisms are operative, namely, solid solution strengthening, grain refinement, precipitation hardening, transformation hardening (bainite strengthening), and dislocation hardening. Due to the multiple mechanisms in operation simultaneously, a process of making an HSLA steel with a ferrite-bainite microstructure must be optimized. Specifically, in order to achieve ultra high strength and excellent ductility, precipitation hardening and low temperature transformation hardening must be optimized.
Conventional HSLA steels have typically been produced at strength levels up to and including 80 ksi minimum yield strength. These steels are conventionally strengthened by a combination of grain refinement and precipitation strengthening requiring the addition of the precipitate forming elements, such as niobium (Nb), titanium (Ti) and vanadium (V), individually or in combination. If a structural application requires a steel with a 110 ksi yield strength, a conventional steel can be strengthened by heat treating processing steps, such as quenching and tempering.
Heat treating processes increase the labor costs, energy expense, and production cycle time associated with the treated steel versus xe2x80x9cas hot rolledxe2x80x9d steel. An HSLA steel which achieves strength levels of 110 ksi and offers the same mechanical properties, without the need for heat treatment, would be advantageous in many applications.
In addition, an HSLA product with increased yield strength could be substituted for a known steel characterized by a lesser yield strength, i.e., 80 ksi. The higher strength HSLA steel product could offer equivalent strength at proportionally reduced thickness. The effect would be to offer steel consumers, such as original equipment manufacturers, equivalent strength steel at reduced weight. This product offering would be beneficial in a variety of weight-sensitive applications, such as automobile design.
The development of an xe2x80x9cas-rolledxe2x80x9d HSLA steel with a yield strength of 110 ksi, sometimes referred to as an xe2x80x9cultra strengthxe2x80x9d HSLA steel, is desired in the steel manufacturing market. Any ultra strength steel developed must be characterized by a combination of strength and toughness, weldability, formability, and fatigue resistance in order to maximize its usage for a variety of applications.
Thus, there is a need in the steel manufacturing market for an HSLA steel characterized by high yield strength, beneficial mechanical properties, and the allowance of low weight components, which is produced by a cost, energy, and time effective method.
The present invention is directed to a method of producing a high-strength low-alloy (HSLA) hot rolled steel having a unique composition of alloying elements and high yield strength.
The resultant steel produced by a method in accordance with the present invention has a yield strength of at least 110 ksi, while offering beneficial mechanical properties of toughness, weldability, formability, and fatigue resistance. The method utilizes an alloying composition with an increased amount of molybdenum in combination with a precisely controlled coiling temperature.
A method of making a high-strength low-alloy steel comprises the first step of hot rolling a steel slab of the following composition (% by weight):
C: 0.03-0.08;
Mn: 1.3-1.8;
Mo: 0.15 to 0.30;
Ti: 0.05-0.10;
B: 0.0005-0.002;
Nb: 0.07-0.11;
Si: up to 0.50;
Al: 0.015-0.10;
S: up to 0.005; and
P: up to 0.03; with the balance being Fe and unavoidable impurities;
The hot rolling step is carried out at an austenitic hot roll finishing temperature. The hot rolled steel is coiled at a temperature ranging from 1120xc2x0 F. to 1180xc2x0 F. The resultant steel is characterized by having a yield strength of at least 110 ksi.
The steel may be further characterized as having a substantially ferrite and bainite microstructure. The volume fraction of bainite is typically 10 to 20%. The method may comprise the step of non-interrupted cooling after the hot rolling step to prevent recrystallization of deformed austenite, thereby increasing the nucleation sites for ferrite and bainite microstructures. The method may further comprise the step of rapid cooling directly after the hot rolling, whereby a fine ferrite grain size is achieved. The ferrite grain diameter is typically 3 to 8 microns.
More specifically, in another embodiment, the first step comprises hot rolling a steel slab of the following composition (% by weight):
C: 0.04-0.06;
Mn: 1.4-1.6;
Mo: 0.18 to 0.22;
Ti: 0.065-0.085;
B: 0.0005-0.001;
Nb: 0.08-0.09;
Si: up to 0.30;
Al: 0.020-0.070;
S: up to 0.005; and
P: up to 0.015; with the balance being substantially Fe and unavoidable impurities;
The hot rolling step is carried out at an austenitic hot roll finishing temperature. The hot rolled steel is coiled at a temperature ranging from 1120xc2x0 F. to 1180xc2x0 F. The resultant steel is characterized by having a ferrite-bainite microstructure and a yield strength of at least 110 ksi. The austenitic hot rolling finishing temperature may range from 1540xc2x0 F. to 1630xc2x0 F.
Many additional features and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows.